What is Root Rot?
“Root rot” is a generic term encompassing any number of soil-inhabiting fungi, or fungi-like organisms that ultimately (and usually quickly) cause the collapse of a plant’s root system. It is a disease that thrives when soils are moist, poorly drained and depending on the particular pathogen, the appropriate temperature to “set up shop.” It often occurs in plants that have a high level of salts from over-fertilization.
The most common types of root rot pathogens are Thielaviopsis, Fusarium, Rhizoctonia, Pythium and Phytopthora. Pythium and Phytopthora are not true fungi, but rather organisms that behave like fungi. This is relevant when seeking treatment options. These are not however the only pathogens that cause root rot. Precise identification is not always important, so long as the conditions that favor disease have been corrected and all of the affected plant material has been removed. If a positive identification of a particular pathogen is required, the diseased plant may be sent to a lab for analysis. Contact your local cooperative extension to learn where the appropriate plant tissue lab is located in your state.
Signs/Symptoms of Root Rot
There are several symptoms of root rot and many can often be mistaken for nutrient deficiencies or temperature extremes. Though there are several fungal pathogens that cause root rot, the symptoms of them are all very similar. Plants displaying signs of root rot are usually wilted and fail to “bounce back” after irrigation. Plants will often appear to be stunted and show no signs of new growth. Leaves, often lower ones, will yellow and may drop off. This is due to the plant’s inability to properly absorb water and nutrients.
If any of these symptoms are noticed, it is important to check the roots. Root systems of healthy plants are usually white or light colored and fibrous, or possess a robust network of large and small tendril-like appendages. Roots that have succumbed to fungal pathogens are brown, often “mushy” and may appear stunted. Fungal pathogens enter through a plant’s feeder roots and then spread throughout the entire root system. This process can occur in as quickly as a few days to a week, or longer. The amount of pathogen, health of the plant, soil moisture and temperature combinations affect the speed of transmission.
How to Treat it
The short answer to this question is, “you don’t.” In the vast majority of cases, once you have observed the foliar symptoms of root rot, it is generally too late to do anything about it. On the outside chance that the early stages of root rot have been observed, there is hope that the affected plant can be nursed back to health.
Remove the plant from the soil and container. Make sure to dispose of all of the soil and prune off as much of the diseased root system as possible. If possible, transplant into a new container. If reusing the old container is the only option, make sure to thoroughly disinfect it with a bleach solution or equivalent such as H2O2. A solution of one part bleach to nine parts water is sufficient to kill pathogens. Make sure that the entire container can be submerged in the solution and allow it to soak for no less than 30 minutes.
When replanting salvageable plants, make sure that the media is clean. If planting back into a soilless media, ensure that it was pasteurized or treated with heat before being sold. If the plants are to go into a hydroponics application, make sure that all of the water has been drained from the system and that clean water with hydrogen peroxide or an equivalent product has been cycled through repeatedly. If plants are to be replanted into the ground, make sure that enough of the contaminated soil has been removed and that the replacement soil has sufficient drainage.
There are available fungicides designed to treat or control root rots. Often though, the disease is discovered after treatment is a viable option. If a fungicide is sought, proper identification of the pathogen is a must and make sure to examine the label carefully. Since Pythium and Phytopthora are not true fungi, not all fungicides will control them; conversely a pesticide designed to control Pythium or Phytopthera, may be impotent when used to fight Thielaviopsis, Fusarium, or Rhizoctonia.
How to Prevent it
Preventing root rot is far easier and a more effective approach than trying to combat it once it has taken hold of your plant’s root system. Overwatering is the most common cause of root rot. Allowing soils to dry out between waterings will go a long way to prevent fungal pathogens. Improving your soil media’s drainage will also help if this is feasible. Selecting an appropriate media for the types of plants or crops you wish to grow will also help reduce the risk of root rot as these soils should contain a proper balance of large and small particles to allow for proper drainage.
Ample light and ventilation will also help to reduce the incidence of root rot. Make sure that your plants receive enough natural or artificial light and air exchange so that excess moisture can be evaporated or dry up before it sets the stage for the introduction of fungal pests. Don’t over-crowd your plants; plan for them to grow. Do this by spacing them far enough apart so that they receive ample air circulation or thin those out once they start to grow into each other.
Avoid over-fertilization of your plants. Too much fertilizer will inhibit your plants’ natural defenses against diseases. Too high a fertilizer value will also burn root tips, which become easy access points for pathogens to avail themselves of. As with any soil amendment, pesticide or fertilizer, make sure to use no more than the manufacturer’s recommendations. When it comes to pesticide use, this is not only best practice, it is also the law.
Finally, make sure to never re-use soil media that has been infected with any pathogen, fungal or otherwise. Hygienic growing practices require the use of new soil with any new planting or transplanting, but on occasion (or frequently) we have all reused soil out of necessity. If soil needs to be re-used, make sure that it has only been in contact with healthy plants.